DE102015009490B4 - Voltage stabilization of a motor vehicle generator - Google Patents

Abstract

Method for operating a generator (3) of a motor vehicle (1), the generator (3) an externally excited electrical machine (7) with an excitation coil (10) and an active rectifier (8) for generating a rectified output voltage (U) one of of the electrical machine (7) generated alternating voltage, in a normal operating mode (M1) the output voltage (U) is regulated to a predetermined target value (18) by an excitation current (le.) depending on an actual value (20) of the output voltage (U) ) is set in the excitation coil (10), characterized in that the output voltage (U) is buffered by a battery (4) of the motor vehicle (1) and decoupling of the battery (4) from the generator (3) is detected and at detected decoupling a battery error signal (23) is generated and depending on the battery error signal (23) in an emergency mode (M2) is changed by the excitation current (le) to one of de m the actual value (20) is set to an independent constant value, the constant value being a maximum value provided in normal operating mode (M1), and regulating the output voltage (U) to the setpoint value (18) by switching the active rectifier (8) by means of a field-oriented control becomes.

Description

The invention relates to a method for operating a generator of a motor vehicle. The generator is in particular a low-voltage generator for a low-voltage vehicle electrical system. The generator comprises an externally excited electrical machine with an excitation coil, the excitation current of which is set in order to regulate an output voltage of the generator to a desired value. The invention also includes a control device for carrying out the method according to the invention and a motor vehicle with the control device according to the invention.

A generator with an externally excited synchronous machine for a motor vehicle is, for example, from the DE 101 16 202 A1 known. The AC voltage generated by the electrical machine of the generator is converted into the rectified output voltage by means of a passive rectifier. The output voltage is 12 volts and is supplied to electrical consumers and a battery of the motor vehicle. The excitation current is set by a regulator depending on the output voltage of the generator, the output voltage of the generator being regulated to approximately 14 volts. The generator also has an active rectifier, by means of which the electrical machine can be used as a starter motor for an automatic start-stop.

From the DE 10 2012 210 907 A1 and the DE 10 2012 222 220 A1 a traction drive system for a motor vehicle is known, which has an electrical machine that is operated by means of a field-oriented control. If a battery for supplying the electrical machine fails, the field-oriented control is carried out further and the electrical machine is thereby supplied with electrical energy, which is stored in a DC voltage intermediate circuit. The electrical machine is a permanently excited synchronous machine which has permanent magnets on the rotor for generating the magnetic excitation.

In today's 12-volt standard generators (also referred to as alternators), voltage regulation takes place via said regulator, which is provided, for example, as a regulator chip in the form of an application-specific integrated circuit (ASIC). The voltage regulation of the electrical machine is done by adjusting the excitation current via a pulse width modulated duty cycle, which is clocked by a switch called an excitation transistor. A diode, which is connected in parallel to the excitation coil, enables the excitation current driven by the inductance of the excitation coil to flow off when the excitation transistor is opened. This diode thus represents a freewheeling diode. The excitation current is fed via sliding contacts into the externally excited synchronous machine, where it generates a magnetic field in the excitation coil located on the rotor. The output voltage of the electrical machine is regulated as a function of the load and speed by means of the magnitude of the flux which arises in the magnetic circuit. The excitation current therefore represents the manipulated variable in the control loop.

The controller dynamics of the generator for setting the output voltage is determined by the electrical time constant T of the excitation coil. The time constant T can be determined as T = L / R, where L is the inductance of the excitation coil and R is its electrical resistance. The time constant T can be in a range from, for example, 130 milliseconds to 180 milliseconds.

The rectification of the AC voltage generated by the electrical machine can be effected by means of a rectifier. This can be a rectifier, for example, which is formed from a bridge circuit, in particular a B6 bridge circuit. For example, tens diodes or actively switched transistors, for example MOSFET (metal-oxide semiconductor field-effect transistor), can be used as rectification valves or switches of the bridge circuit.

Due to the electrical time constant T of the excitation coil, that is to say the described regulator dynamics of the generator, this arrangement can only react to a sudden increase in the electrical power required as quickly as the time constant T permits. In the case of dynamic or erratic processes, for example a change in the speed of the mechanical drive for the generator or a change in the load of the electrical load supplied by the generator, the electrical power must first be provided by the battery of the motor vehicle until the excitation current returns to the appropriate value is settled. If the battery is faulty or an electrical connection between the generator and the battery is interrupted, it can happen in one of the dynamic processes described that the power supply of the electrical consumers breaks down, that is to say the output voltage drops below a predetermined minimum value, so that, for example, control units no longer meet specifications function. It can also happen that the excitation current comes to a standstill in the excitation coil and the generator then no longer generates any electrical voltage at all.

In the DE 198 49 889 A1 a control for an electrical machine is described in which for low speeds, an excitation current of an excitation winding of a synchronous machine is set to a maximum value and the power output of the electrical machine is set by a field-oriented control. As soon as the speed is high enough, a switch is made to excitation current control.

From the DE 10 2006 044 426 A1 A method for stabilizing the operating voltage in a vehicle electrical system is known, by means of which the electrical system is additionally supported by the generator in the case of a low-power battery. For this purpose, the control dynamics of the generator are increased. To increase the control dynamics, a clock duty cycle of the control and a time constant of a digital filter are changed.

From the US 2006/0232247 A1 a method for monitoring an electrical connection between a generator and a battery is known. If the electrical connection is faulty, a voltage control of the generator is switched over, since it would otherwise produce an overvoltage. In normal operation as well as in the case of a fault described, an excitation current control is carried out, only the target voltage being changed over. The excitation current is set by alternately switching the excitation current on and off.

From the US 2013/0207589 A1 a method for controlling a torque of an electrical machine by means of a field-oriented control is known. The electrical machine is excited with a constant excitation current. The field-oriented control is chosen because this is the best way to overcome the moment of inertia of the rotor.

The invention has for its object to ensure stable operation of a motor vehicle generator with an externally excited electrical machine.

The object is achieved by the subject matter of the independent claims. Advantageous developments of the invention result from the features of the dependent claims.

The invention provides a method for operating a generator of a motor vehicle. The method assumes that the generator has an externally excited electrical machine with an excitation coil and an active rectifier for generating a rectified output voltage of the AC voltage generated by the electrical machine. The method provides two operating modes for the generator, a normal operating mode and an emergency operating mode. In normal operating mode, the output voltage is regulated to a predetermined setpoint value by setting an excitation current in the excitation coil as a function of an actual value of the output voltage. Normal operation has the advantage that the generator is operated in a very energy-efficient manner because the current through the excitation coil is set as low as possible in order to set the setpoint for the output voltage.

In normal operating mode, it is necessary that the generator can be buffered by a battery: in the event of a sudden change in the mechanical drive power for the generator or a sudden change in the electrical power required by the connected electrical consumers, the battery ensures the supply to the consumers for as long as this until the excitation current is regulated to the appropriate setpoint again.

According to the invention, the output voltage is thus buffered or supported in the manner described by a battery of the motor vehicle. The invention additionally provides that a decoupling of the battery is recognized by the generator and a battery error signal is generated when a decoupling is detected. The system then switches from the normal operating mode to the emergency operating mode as soon as the battery is decoupled from the generator. The decoupling is in particular a galvanic decoupling.

If the battery is no longer available for this buffering, the system switches to emergency mode. For this purpose, the method provides that a change is made to the emergency operating mode as a function of the battery error signal. The battery error signal indicates that the battery is electrically isolated or decoupled from the generator. A switch is made to the emergency operating mode in that the excitation current is set to a constant value that is independent of the actual value of the output voltage and the output voltage is regulated to the target value by switching the active rectifier by means of a field-oriented regulation. In other words, a constant magnetic excitation is set in the electrical machine, which is achieved by the constant excitation current. In order to continue to regulate the output voltage to the setpoint, a field-oriented regulation for the generator is carried out instead of the excitation current regulation. The field-oriented control means that the rectifier valves, for example the transistors, are switched in the active rectifier as a function of a rotational position of the rotor and as a function of the actual value of the output voltage.

The invention has the advantage that in normal operating mode, when the battery is available for buffering the output voltage, energy-efficient operation with an adapted excitation current is carried out, while in emergency operating mode, when the battery is not available for buffering, on the basis of the field-oriented Regulation The output voltage is regulated to the setpoint, which can achieve control dynamics that are independent of the time constant of the excitation winding. The invention is particularly provided for operating a low-voltage generator for a low-voltage electrical system of a motor vehicle. Low voltage is understood to mean an electrical voltage less than 60 volts, in particular less than 20 volts. To determine the rotational position of the rotor, a rotational position sensor for the rotor can be provided in a manner known per se, as is used in connection with field-oriented control in the prior art.

According to the invention, the excitation current is set to a maximum value for the emergency operating mode, as is also provided as the maximum value for the normal operating mode. This has the advantage that it is possible to react to every possible extreme case even in the emergency operating mode. An energy efficiency of the generator is reduced as a result, since the excitation current is set permanently or continuously with the maximum value. However, the motor vehicle can also be operated in the emergency operating mode without restriction.

The invention also includes optional further developments, the features of which result in additional advantages.

According to a further development, a time constant of a controller dynamics of the field-oriented control is smaller than the time constant of the excitation coil. The time constant of the controller dynamics of the field-oriented control can be determined, for example, in the same way as the time constant of the excitation coil, in that a mechanical drive power for the generator or the electrical power tapped by the electrical consumers at a DC voltage output of the generator is changed in a step-wise or step-wise manner. The field-oriented control then reacts by adapting the output voltage. The time constant can then be determined, for example, by determining a period of time required by the controller to regulate a deviation of the output voltage from the setpoint from the time of the jump to a value 1 / e, where e is Euler's number (e = 2 , 71828 ...). The further development has the advantage that the sudden change in the mechanical drive power and / or the electrical power of the consumers can be compensated or compensated for by means of the generator alone in the emergency operating mode by means of the field-oriented control, so that no battery is required for the described buffering.

According to a further development, it can be provided for a further stabilization that at least one predetermined consumer, electrically coupled to the generator, is switched off for the emergency operating mode. Then this consumer can no longer cause a change in the electrical power required.

According to a development, the active rectifier is used to rectify the AC voltage in normal operating mode by emulating a diode-based passive rectifier by switching the rectifier. In other words, switching times of switches of the rectifier, that is to say for example of transistors, are selected in such a way that they would also result if the rectifier were designed as a passive rectifier with diodes. This has the advantage that the active rectifier does not influence the regulation of the excitation current. A conventional regulator chip known from the prior art can thus be used for regulating the excitation current in the normal operating mode.

In order to recognize the decoupling, a further development provides that a battery current of the battery is determined and the decoupling is recognized if the amount of the battery current for a predetermined period of time is less than a predetermined first threshold value and / or if the setpoint for the Output voltage no equalizing current flows between the battery and the generator. These two indications can only occur if the battery is decoupled from the generator. They are therefore a reliable identifier for the need to switch to emergency mode.

In addition or as an alternative to this, the decoupling can be recognized by detecting a time profile of the output voltage. The decoupling is recognized here if a ripple in the output voltage is greater than a predetermined second threshold value. In other words, it is detected that the buffering caused by the battery is no longer present because a fluctuation in the amplitude of the output voltage leaves a predetermined tolerance band in which the setpoint lies. The ripple can be determined, for example, by subtracting a temporal mean value from a time signal of the voltage value of the output voltage and then detecting a peak-to-peak amplitude as ripple. This further development has the advantage that the decoupling already occurs on the generator side too can be detected without a battery current having to be detected on the battery.

As already stated, the invention also includes a control device for a generator. The control device can be configured, for example, as a control device. The control device has a first control interface for a switching element of an excitation coil of an externally excited electrical machine of the generator and a second control interface for switching elements of an active rectifier of the generator. In other words, the control device can switch both the excitation coil and the active rectifier. The control device is designed or set up to carry out an embodiment of the method according to the invention. For this purpose, the control device can have, for example, a microcontroller or a microprocessor.

Finally, the invention also includes a motor vehicle with a generator and with a battery coupled to the generator. In the motor vehicle according to the invention, an embodiment of the control device according to the invention is provided, which is coupled to the generator. The motor vehicle according to the invention is preferably designed as a motor vehicle, in particular as a passenger car.

An exemplary embodiment of the invention is described below. For this purpose, the single figure (FIG.) Shows a schematic representation of an embodiment of the motor vehicle according to the invention.

The exemplary embodiment explained below is a preferred embodiment of the invention. In the exemplary embodiment, the described components of the embodiment each represent individual features of the invention that are to be considered independently of one another, which further develop the invention independently of one another and are therefore also to be regarded individually or in a combination other than the one shown as part of the invention. Furthermore, the described embodiment can also be supplemented by further features of the invention that have already been described.

The figure shows a motor vehicle 1 , which can be, for example, a motor vehicle, in particular a passenger car. An electrical system is shown 2nd with a generator 3rd , a battery 4th and electrical consumers 5 , the consumer 5 are symbolically represented only by a single element. The generator 3rd generated at DC voltage connections 6 a DC voltage U that is less than 60 volts, especially the DC voltage U less than 20 volts, in particular it is in a range between 10 volts and 15 volts. With the electrical system 2nd can therefore be a 12-volt electrical system. With the battery 4th it can be, for example, a lead accumulator.

To generate the DC voltage U instructs the generator 3rd an electrical machine 7 and an active rectifier 8th on. The DC voltage U is between a plus line B + and a ground line GND of the electrical system 2nd generated. On the two lines B + , GND are also the battery 4th and the consumer 5 connected.

With the electrical machine 7 it is an externally excited electrical machine, in particular an externally excited synchronous machine. The electrical machine 7 has a rotor 9 with an excitation coil 10th and a stator 11 with stator coils 12th on. The electrical machine 7 can be designed in a manner known per se.

The rectifier 8th is an active rectifier with half bridges 13 about which respective winding ends 14 of the stator coils 12th both with the plus line B + as well as the ground line GND each via a switching element 15 are connected. For the sake of clarity, only some of the switching elements are shown in the figure 15 provided with a reference number. The switching elements 15 are controllable. It can be the switching elements 15 act, for example, transistors, in particular MOSFET. The switching elements 15 are controlled by a control unit 16 controlled.

An excitation current le through the excitation coil 10th can by a first voltage regulator 17th be regulated, which is the DC voltage U to a setpoint 18th adjusts by means of an excitation transistor 19th by clocking or by pulsed switching of the excitation transistor 19th the average current strength of the excitation current le is set such that one of the first regulator 17th determined actual value 20th the DC voltage U the setpoint 18th corresponds. The excitation transistor 19th represents a switching element of the excitation coil 10th A free-wheeling diode can also be used for clocked switching 21 for the excitation coil 10th be provided.

The regulator operation by means of the first voltage regulator 17th , ie the excitation current control, is used in the motor vehicle 1 in a normal operating mode M1 performed when the battery 4th also an electrical connection to the two lines B + , GND having. This is done by a monitoring device 22 monitors, for example, a battery current in the manner described Ib monitors or a ripple of the DC voltage U supervised. The monitoring device 22 can be implemented by an electrical circuit. If the monitoring device 22 an interruption in the electrical contact of the battery 4th to the lines B + , GND recognizes, it signals this with a battery error signal 23 .

The battery error signal 23 is sent to the control unit 16 transmitted. The control unit 16 then changes from the normal operating mode M1 in an emergency operating mode M2 . In other words, the first voltage regulator 17th deactivated. The voltage regulator 17th closes the excitation transistor 19th , that is the excitation transistor 19th is in emergency mode M2 in a permanently conductive state. As a result, the excitation coil 10th flowed through or energized with the maximum possible value for the excitation current le.

The control unit 16 has a second voltage regulator 24th on, which also regulates the output voltage U to the setpoint 18th carries out. The control unit 16 can have, for example, a microcontroller or a microprocessor. An actual value 20th the output voltage U can by a voltage detection device 25th can be determined which is the actual value 20th to the control unit 16 transmitted. The voltage detection device 25th can be designed in a manner known per se.

The second voltage regulator 24th controls the switching elements 15 of the rectifier 8th . Through the voltage regulator 24th can each of the switching elements 15 independent of the other switching elements 15 can be controlled. This is a field-oriented regulation for the generator 3rd performed to the output voltage U to the setpoint 18th to settle. A corresponding field-oriented regulation is known per se from the prior art.

The control unit 16 , the voltage regulator 17th and the voltage detector 25th thus put together a control device C. in the sense of the invention.

To go from the emergency mode M2 back to normal operating mode M1 To change, it can be provided that this is brought about by a control device or diagnostic device external to the vehicle, which can be connected to the motor vehicle in a workshop, for example.

If in the motor vehicle 1 only the first voltage regulator 17th would exist for the excitation current-based control, this combination of bridge rectifier and standard controller would have the disadvantage of dynamic processes, such as those that occur, for example, when the speed of a mechanical drive of the rotor changes 9 or in the event of an electrical load change in the consumer 5 could result. In battery-free operation, i.e. when the electrical connection between the battery 4th and the lines B + , GND would be interrupted, dynamic driving maneuvers, such as a combination of steering intervention, braking intervention and chassis intervention, would be a fluctuation in the power supply voltage, that is to say the output voltage U , strong and it is even possible that the specified voltage limits for the output voltage U get hurt. The reason for this lies in the described time constant of the excitation circuit with the excitation coil 10th , which prevents the excitation current le from being tracked with sufficient speed or dynamics.

In the motor vehicle 1 can now this dynamic with the rectifier 8th be created with the field-oriented regulation. The time constant of a field-oriented control, which is only the switching elements 15 , that is to say in particular transistors, must therefore be of the order of magnitude of the switching time of such transistors. This requires the electrical machine 7 are under constant full excitation, which adversely affects efficiency, but this operation is only possible for battery-free emergency operation. There is also no need to use expensive permanent magnets.

The combination of field-oriented control and excitation current control can be used to create a control strategy for battery-free operation, which involves dynamic voltage control of the output voltage U enabled without loss of efficiency during normal operation. There is also no need to provide expensive permanent magnets.

Thus, in normal operation, the rectifier works with an actuation method that detects the diode behavior of the switching elements 15 mimicking. The output voltage U is controlled by the controller 17th set for the excitation current le. The efficiency of the electrical machine 7 is therefore optimal since the full excitation current le does not flow in partial load operation. The excitation current represents a leakage current. However, the machine dynamics are limited by the excitation time constant.

Should now be in the vehicle electrical system 2nd battery-free operation can be detected by the monitoring device 22 is signaled by means of the battery error signal, is from the normal operating mode of the generator 3rd switched to emergency mode. The diagnosis can be, for example, the battery diagnosis described or a sensation of the voltage ripple on the generator 3rd be yourself.

The voltage regulator 17th then sets the maximum excitation current le, which causes the maximum magnetic flux in the electrical machine 7 arises. The field-oriented regulation in the rectifier is used for voltage regulation 8th used. This type of control is more dynamic than the excitation current control. Operating point changes caused by the described dynamic processes, for example dynamic driving maneuvers, can be achieved by appropriately fast activation of the switching elements 15 and take place very quickly due to the short switching times of these switching elements. Thus the output voltage U can be readjusted much faster during dynamic driving maneuvers. In emergency operation mode, the efficiency of the machine is worse because the high excitation current le always flows. However, this is offset by the advantage that the output voltage remains stabilized U is also provided in battery-free operation.

Overall, the example shows how the invention enables voltage stabilization of the 12V generator to be provided by a combination of field-oriented control and excitation current control.

Claims (8)

Method for operating a generator (3) of a motor vehicle (1), the generator (3) an externally excited electrical machine (7) with an excitation coil (10) and an active rectifier (8) for generating a rectified output voltage (U) one of of the electrical machine (7) generated alternating voltage, in a normal operating mode (M1) the output voltage (U) is regulated to a predetermined target value (18) by an excitation current (le.) depending on an actual value (20) of the output voltage (U) ) is set in the excitation coil (10), characterized in that the output voltage (U) is buffered by a battery (4) of the motor vehicle (1) and decoupling of the battery (4) from the generator (3) is detected and at detected decoupling a battery error signal (23) is generated and depending on the battery error signal (23) is switched to an emergency mode (M2) by the excitation current (le) to one of d an actual value (20) independent constant value is set, the constant value being a maximum value provided in normal operating mode (M1), and the output voltage (U) regulated to the setpoint value (18) by switching the active rectifier (8) by means of a field-oriented control becomes. Procedure according to Claim 1 , wherein a time constant of a controller dynamics of the field-oriented control is smaller than a time constant of the excitation coil. Method according to one of the preceding claims, wherein in the normal operating mode (M1) the active rectifier (8) is used to rectify the AC voltage in that a diode-based passive rectifier is emulated by switching the rectifier (8). Method according to one of the preceding claims, wherein a battery current (Ib) of the battery (4) is determined and the decoupling is detected if the amount of the battery current (Ib) is smaller than a predetermined first threshold value for a predetermined period of time and / or if Changing the setpoint (18) for the output voltage (U) no equalizing current flows between the battery (4) and the generator (3). Method according to one of the preceding claims, wherein a time profile of the output voltage (U) is detected and the decoupling is recognized if a ripple of the output voltage (U) is greater than a predetermined second threshold value. Method according to one of the preceding claims, wherein at least one predetermined consumer (5) electrically coupled to the generator (3) is switched off for the emergency operating mode (M2). Control device (C) for a generator (3), the control device (C) having a first control interface for a switching element (19) of an excitation coil (10) of a separately excited electrical machine (7) of the generator (3) and a second control interface for switching elements ( 15) of an active rectifier (8) of the generator (3), characterized in that the control device (C) is designed to carry out a method according to one of the preceding claims. Motor vehicle (1) with a generator (3) and with a battery (4) coupled to the generator (3), characterized in that the motor vehicle (4) has a control device (C) Claim 7 having.

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